Method for desensitizing offset printing plates

ABSTRACT

A method for desensitizing an offset printing plate which comprises applying an aqueous desensitizing composition consisting essentially of at least one complex to a non-image area of the offset printing plate having an oleophilic image formed thereon in an amount effective to desensitize said non-image area.

This is a division of application Ser. No. 878,400, filed Feb. 16, 1978, U.S. Pat. No. 4,208,212.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a method for desensitizing offset printing plates with an aqueous treating liquid.

At present, as offset printing plates, there are known an electrophotographic plate which is provided with a photosensitive layer formed by dispersing inorganic photoconductive particles, such as zinc oxide particles, in a resinous binder and it is intended to form an hydrophobic image thereon by an electrophotographic process, a direct image-printing plate which is provided with an image-accepting layer formed by dispersing an inorganic pigment, such as titanium oxide, in a resinous binder and it is intended to form an image on said layer by directly writing thereon with oily ink or typewriting, a P S plate which is provided with a photosensitive layer consisting of a photohardening resin on an aluminum plate with a coarsened surface and it is intended to form an image by utilizing the difference between the solubility of the exposed area and that of the non-exposed area of said photosensitive layer, and so forth. All of these plates are usually made into an offset master by forming an oleophilic image thereon and then subjecting same to a desensitizing treatment for making the non-image area of the plate hydrophilic. The treating liquid for use in this desensitizing treatment can be broadly divided into 3 kinds: one which consists essentially of a hydrophilic resin such as gum arabic and polyvinyl pyrrolidone or at least one member selected from the group consisting of phosphate, aluminum-alum compound and acid (inorganic or organic), one which consists essentially of a ferrocyanide or ferricyanide proposed in U.S. Pat. No. 3,001,872, and one which comprises phytic acid or a metal salt of phytic acid disclosed in Japanese Patent Publication No. 24609/1970 and Japanese Patent Open No. 103501/1976. However, these treating liquids leave something to be desired for use as a satisfactory treating liquid. To be concrete, the first treating liquid is not capable of forming a hydrophilic film having a high physical strength on the non-image area and its film-forming speed is low, and accordingly, when an offset master treated with such an aqueous liquid is employed for printing, the master and the resulting prints develop stains of gearstripe (upon suddenly rotating a printing cylinder at the beginning of offset printing, a blanket cylinder rubs the surface of an offset master thereby to deteriorate the desensitized surface of the same with printing stains.), stains on the ground and collapse of the image upon turning out prints in small quantities, so that it is not of practical use. The second processing liquid, as compared with the first treating liquid, has such merits that it is superior in desensitizability and the physical strength of the hydrophilic film formed thereof is high and the film-forming speed is high. However, it is defective in that it becomes colored when subjected to light or heat, or it gives rise to precipitates while in use or in storage, thereby making the desensitizability thereof unstable. Not only that, as it contains cyan ions, it is undesirable from the view point of public nuisance. And, the third processing liquid is defective in that it is unsatisfactory in respect of desensitizability, and it gives rise to precipitates with the passing of time, thereby causing deterioration of the desensitizability thereof.

SUMMARY OF THE INVENTION

The present invention is intended to provide a method for desensitizing offset printing plates with a cyanless treating liquid which has an intense desensitizability, is capable of rapidly forming a firm hydrophilic film, is free from deterioration of the efficiency thereof when subjected to light or heat, and poses no problem of public nuisance.

The present invention is also intended to provide a practical treating liquid for use in offset printing which is so superior in durability in printing that there occur no stains of gear-stripe, stains on the ground or collapse of the image on the offset master or prints even in turning out a lot of prints.

The present invention relates to an aqueous treating liquid for use in offset printing, which comprises at least one member selected from the group consisting of compounds expressed by the general formula I [M(X₁)a](Y)b.cH₂ O (wherein M represents a metal of divalence or more, X₁ represents NH₃, OH₂, H₂ N(CH₂)₂ NH₂, C₂ O₄, NO, NO₂, OCHO, NH₂, HONC(CH₃)C, (CH₃)NO, ##STR1## OCN₂ H₄ or OC(NH₂)₂, Y represents anion, a is a number ranging from 2 to 6, b is a number ranging from 1 to 3, and c is 0 or a number ranging from 1 to 10), compounds expressed by the general formula II [M(X₁)a'(X₂)a"](Y)b.cH₂ O (wherein M, X₁, Y, b and c are respectively the same as that in the general formula I, X₂ represents OH, OH₂, NO₂, CO₃, NH₂ CH₂ COO, HONC(CH₃)C(CH₃)NO, Br, Cl, H₂ N(CH₂)₂ NH₂, ONO₂, ONO, NCS, H₂ O, N, ##STR2## F or I, and a' and a" are respectively a number ranging from 1 to 5), compounds expressed by the general formula III (M₁)p[(M₂)(X₃)q].nH₂ O (wherein M₁ represents Na, K, NH₄ or hydrogen atom, M₂ represents a metal of divalence or more, X₃ represents C₂ O₄, NO₂, Cl, Br, I or ##STR3## p is a number ranging from 1 to 3, q is a number ranging from 1 to 6, and n is 0 or a number ranging from 1 to 10), compounds expressed by the general formula IV (M₁)p[(M₂)(X₃)q(X₄)r].nH₂ O (wherein M₁, M₂, X₃, p, q and n are respectively the same as that in the general formula III, X₄ represents NH₃ or NH₂ CH₂ CH₂ NH₂, and r is a number ranging from 1 to 6) and compounds expressed by the general formula V (M₁)p[(M₂)(X₃)q(X₄)r(X₅)s].nH₂ O (wherein M₁, M₂, X₃, p, q and n are respectively the same as that in the general formula III, X₄ and r are respectively the same as that in the general formula IV, X₅ represents C₂ O₄, NO₂, Cl or Br, s is a number ranging from 1 to 6). In short, the present invention relates to an aqueous treating liquid comprising at least one member selected from compounds expressed by the general formula I, compounds expressed by the general formula II, compounds expressed by the general formula III, compounds expressed by the general formula IV or compounds expressed by the general formula V.

In this context, to give concrete examples of M or M₂ in the general formulas I through V, there are Zn, Ir, Co, Ti, Fe, Cu, Ni, Pt, Mn, Ru, Rh, Hf, V, Be, etc., and to give concrete examples of Y in the general formulas I and II, there are I, Br, Cl, Cl₃, Cl₄, C₂ O₄, SO₄, NO₃, NO₂, CH₃, COO, HCOO, BF₄, MnO₄, OH, F, HSO₄, HPO₄, PO₄, HPO₃, SO₄ X (wherein X represents Cl, Br, I, ClO₄ or NO₃), etc.

Complexes useful for the present invention which are expressed by the foregoing general formulas form a desensitizing salt which is very firm, stable and hard to dissolve in water in the presence of metal ions. Moreover, these complexes are stable against light and heat and, accordingly, are not only free from deterioration of the desensitizability thereof with the passing of time but also capable of forming a desensitizing film which is firmer and stabler than that formed of any cyan compound. Besides, inasmuch as these complexes contain no cyan ions, they pose no problem of public nuisance. Further, while cyan compounds display desensitization effect only in the acid region, complexes according to the present invention display desensitization effect in a wide range extending from acid region to alkaline region.

To give concrete examples of compounds expressed by the foregoing general formulas I and II, there are hexamine cobalt salt like [Co(NH₃)₆ ]Cl₃ and [Fe(NH₃)₆ ]I₂, [Ti(NH₃)₄ Cl₂ ]Cl, [Mn(NH₃)₆ ]Cl₂, [Co(NH₃)₅ H₂ O]Br₃, [Ru(NH₃)₆ ](SO₄)₁.5.2.5H₂ O, {Cu[H₂ N(CH₂)₂ NH₂ ]₃ }Cl₃, [Pt(NH₃)₆ ](OH)₄, [Ni(NH₃)₆ ](ClO₃)₂, [Co(NH₃)₄ Cl₂ ]Cl, [Fe(NH₃)₅ NO₂ ]Cl₂, [Co(NH₃)₅ (OH₂)](C₂ O₄)₁.5.2H₂ O, [Ni(NH₃)₆ ](ClO₃)₂, [Co(NH₃)₄ (NO₂)₂ ]Cl, [Mn(NH₃)₆ ]Cl₃, [Fe(NH₃)₆ ]I₂, etc. And, to give concrete examples of compounds expressed by the general formulas III through V, there are K[Co(NH₃)₂ (NO₂)₄ ], Na[Co(NH₃)₂ (NO₂)₄ ], NH₄ [Co(NH₃)₂ (NO₂)₄ ], K[Co(NH₃)₂ (NO₂)₂ (C₂ O₄)].H₂ O, Na[Co(NH₃)₂ (NO₂)₂ (C₂ O₄)].H₂ O, NH₄ [Co(NH₃)₂ (NO₂)₂ (C₂ O₄)].H₂ O, Na₃ [Co(C₂ O₄)₃ ], (NH₄)₃ [Co(C₂ O₄)₃ ], K₃ [Co(C₂ O₄)₃ ], Na₃ [Co(NO₂)₆ ], (NH₄)₃ [Co(NO₂)₆ ], K[Co(edta)], Na[Co(edta)], (NH₄)[Co(edta)], K₃ [CoCl₆ ], Na₃ [CoCl₆ ], (NH₄)₃ [CoCl₆ ], K₃ [CoBr₆ ], Na₃ [CoBr₆ ], (NH₄)₃ [CoBr₆ ], K[Co(NH₂ CH₂ CH₂ NH₂)(NO₂)₄ ], Na[Co(NH₂ CH₂ CH₂ NH₂)(NO₂)₄ ], K[Co(NH₂ CH₂ CH₂ NH₂)₂ (NO₂)₂ ], Na[Co(NH₂ CH₂ CH₂ NH₂)₂ (NO₂)₂ ], NH₄ [Co(NH₂ CH₂ CH₂ NH₂)₂ (NO₂)₂ ], K₃ [Ni(C₂ O₄)₃ ], Na₃ [Ni(C₂ O₄)₃ ], (NH₄)₃ [Ni(NO₂)₆ ], K₂ [Ni(edta)], Na₂ [Ni(edta)], (NH₄)₂ [Ni(edta)], K₂ [Fe(edta)], Na[Fe(edta)], (NH₄)[Fe(edta)], K₃ [Fe(C₂ O₄)₃ ], Na₃ [Fe(C₂ O₄)₃ ], (NH₄)₃ [Fe(C₂ O₄)₃ ], Pt[ Pt(NH₃)₄ Cl₂ ], H₂ [PtCl₆ ], K₂ [PtCl₆ ], K₂ [PtI₆ ], H₂ [Pt(NO₂)₄ ], NH₄ [Co(NH₂ CH₂ CH₂ NH₂)(NO₂)₄ ], K₂ [Pt(NO₂)₄ ], Na₂ (Pt(NO₂)₄ ], K₂ [Pt(C₂ O₄)₂ ], Na₂ [Pt(C₂ O₄)₂ ], (NH₄)₂ [Pt(C₂ O₄)₂ ], K₂ [Pd(NO₂)₄ ], Na₂ [Pd(NO₂)₄ ], (NH₄)₂ [Pd(NO₂)₄ ], K₂ [Pd(C₂ O₄)₂ ], Na₂ [Pd(C₂ O₄)₂ ], (NH₄)₂ [Pd(C₂ O₄)₂ ], NH₄ [Co(NH₃)₂ (C₂ O₄)(NH₂ CH₂ CH₂ NH₂)], Na[Co(NH₃)₂ (C₂ O₄)(NO₂)₂ ], NH₃ [Co(NH₃)₂ (C₂ O₄)(NO₂)₂ ], etc. In this context, "edta" is an abbreviation of ethylene diamine tetraacetic acid radical ##STR4## These complexes are easily obtained through the known synthesizing process or available on the market. For use in the present invention, among the foregoing compounds, hexamine cobalt salt is especially desirable.

The compounds expressed by the general formulas I through V can be admixed with those substances which are generally employed as assistants to processing liquids. These assistants include, for instance, phosphate, alkali, ammonia, organic salt, amine, etc. as base; fatty acid, aromatic oxycarboxylic acid, inorganic acid (e.g., phosphoric acid) as acid; sulfate, nitrate, etc. as metallic salt; glycerine, alcohol, glycol, natural or synthetic hydrophilic polymer, etc. as wetting agent; aminocarboxylic acid, polyphosphoric acid as antioxidant; and dehydroacetic acid, salicylic acid, etc. as antiseptics. Among these assistants, application of base and/or inorganic acid, especially phosphate and/or phosphoric acid, is desirable.

As will be understood from the foregoing descriptions, a preferable embodiment of the present invention is an aqueous treating liquid comprising hexamine cobalt salt and phosphate and/or phosphoric acid. To be more precise, this processing liquid has an excellent durability in printing, that is, it brings on no stains of gear-stripe or stains on the ground on the offset masters or prints even when used in producing a lot of prints. Hexamine compounds as set forth above are complexes having an isometric octahedral coordination structure. This coordination structure is akin to that of hexacyano compounds such as ferrocyanides, etc. Therefore, hexamine compounds form a very firm and stable desensitizing complex which is hard to dissolve in water upon reacting with metallic ions. Not only that, hexamine compounds are stable against heat and light unlike hexacyano compounds and, accordingly, they are free from deterioration of the desensitizability with the passing of time and capable of forming a desensitizing film which is firmer and stabler than that formed of hexacyano compounds. Besides, while hexacyano compounds display a desensitizing effect only in an acid region, hexamine compounds display that effect in a wide range covering the acid region and alkaline region. Moreover, a desensitizing film (salt) formed of a hexamine compound alone has a sufficient water-holding property (this water-holding property, or the degree of getting wet with water, is expressed by the contact angle between the film and water, and it is considered that the narrower is this contact angle, the better is the water-holding property; in the case of a desensitizing salt of hexamine compound, this contact angle is about 45°), entailing a satisfactory ink-separating property. In the preferable embodiments of the present invention, for the sake of further enhancement of this water-holding property of the desensitizing salt, phosphoric acid and/or phosphate employed jointly with hexamine compounds. In this connection, phosphoric acid or phosphate is admittedly poor in desensitizability as described above when employed independently, but it can form a desensitizing salt having a satisfactory water-holding property (contact angle for water: about 15°) upon reacting with metal ions. On this occasion, the hexamine compound is combined with phosphoric acid and/or phosphate within an aqueous solution and assumes a structure wherein phosphoric acid ions are coordinated on the outside of complex ions. For instance, in the case where [Co(NH₃)₆ ]Cl₃ is combined with Na₂ HPO₄, the hexamine compound assumes the structure {[Co(NH₃)₆ ](HPO₄)₄ }⁵⁻, and this forms a desensitizing salt upon reacting with metal ions. Because the hexamine compound thus forms a desensitizing film which contains HPO₄ ⁻ having a satisfactory water-holding property in the presence of phosphoric acid (or phosphate), the ink-separating property thereof is very much improved.

As examples of hexamine cobalt salt, in addition to the foregoing [Co(NH₃)₆ ]Cl₃, there can be cited [Co(NH₃)₆ ](HPO₄)₃.4H₂ O, [Co(NH₃)₆ ]PO₄.4H₂ O, [Co(NH₃)₆ ].(ClO₄)₃, [Co(NH₃)₆ ](OH)₃.6H₂ O, [Co(NH₃)₆ ]SO₄, [Co(NH₃)₆ ]Br, [Co(NH₃)₆ ](NO₃)₃, [Co(NH₃)₆ ]I₃, [Co(NH₃)₆ ]F₃, [Co(NH₃)₆ ](CF₃ COO)₃, [Co(NH₃)₆ ](CCl₃ COO)₃ [Co(NH₃)₆ ](ClO₃)₃, [Co(NH₃)₆ ]SO₄ Cl, [Co(NH₃)₆ ](TiCl₆), [Co(NH₃)₆ ](BiCl₆), etc.

As phosphoric acid or phosphate, phosphoric acid, metaphosphoric acid, hexaphosphoric acid, trimetaphosphoric acid, dodecaoxo-6-phosphoric acid, hypophosphoric acid, monoammonium phosphate, diammonium phosphate, triammonium phosphate, monosodium phosphate, disodium phosphate, trisodium phosphate, monopotassium phosphate, dipotassium phosphate, tripotassium phosphate, phosphomolybdic acid, sodium pyrophosphate, ammonium phosphomolybdate, monocalcium phosphate, monomagnessium phosphate, sodium ammonium phosphate, imidometaphosphoric acid, calcium pyrophosphate, etc. are useful.

The appropriate amount of these phosphoric acids and/or phosphates to be employed is in the range of from 0.1 to 20 parts by weight, preferably from 1 to 5 parts by weight, per 1 part by weight of hexamine cobalt salt.

The treating liquid of the present invention is applied to the surface of various conventional offset printing plates, such as electrophotographic printing plate, direct image-printing plate, P S printing plate, etc. at a concentration of preferably 0.1 to 30 wt.%.

The treating liquid of the present invention is also useful as wetting solution at the time of offset printing. On this occasion, the treating liquid is diluted with water of 1 to 10 times the quantity thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

    ______________________________________                                         [Co(NH.sub.3).sub.6 ]Cl.sub.3                                                                           50     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 2

    ______________________________________                                         [Fe(NH.sub.3).sub.6 ]I.sub.2                                                                            30     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 3

    ______________________________________                                         [Ti(NH.sub.3).sub.4 Cl.sub.2 ]Cl                                                                        10     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 4

    ______________________________________                                         [Mn(NH.sub.3).sub.6 ]Cl.sub.2                                                                           10     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 5

    ______________________________________                                         [Co(NH.sub.3).sub.5 H.sub.2 O]Br.sub.3                                                                  10     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 6

    ______________________________________                                         [Ru(NH.sub.3).sub.6 ].sub.2 (SO.sub.4) . 5H.sub.2 O                                                     20     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 7

    ______________________________________                                         {Cu[H.sub.2 N(CH.sub.2).sub.2 NH.sub.2 ].sub.3 }Cl.sub.3                                                5      g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 8

    ______________________________________                                         [Pt(NH.sub.3).sub.6 ](OH).sub.4                                                                         20     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 9

    ______________________________________                                         [Ni(NH.sub.3).sub.6 ](ClO.sub.3).sub.2                                                                  2      g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 10

After adding 60 g of (NH₄)₂ HPO₄ to the prescription in Example 1, by further adding citric acid thereto, the pH value was adjusted to be 5.0.

EXAMPLE 11

After adding 60 g of glycerine and 1 g of sodium dehydroacetate to the prescription in Example 2, by further adding malonic acid thereto, the pH value was adjusted to be 6.0.

EXAMPLE 12

    ______________________________________                                         [Co(NH.sub.3).sub.4 Cl.sub.2 ]Cl                                                                        2      g                                              Na.sub.2 PO.sub.4        20     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 13

    ______________________________________                                         [Fe(NH.sub.3).sub.5 NO.sub.2 ]Cl.sub.2                                                                  5      g                                              NH.sub.4 H.sub.2 PO.sub.4                                                                               30     g                                              adipic acid              10     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 14

    ______________________________________                                         [Co(NH.sub.3).sub.5 (OH.sub.2)](C.sub.2 O.sub.4).sub.1.5 . 2H.sub.2                                     5      g                                              tartaric acid            10     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 15

    ______________________________________                                         [Ni(NH.sub.3).sub.6 ] (ClO.sub.3).sub.2                                                                 5      g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 16

    ______________________________________                                         [Co(NH.sub.3).sub.4 (NO.sub.2).sub.2 ]Cl                                                                5      g                                              sodium dehydroacetate    1      g                                              glycolic acid            10     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 17

After adding 50 g of Na₃ PO₄ to the prescription in Example 3, by further adding phosphoric acid thereto, the pH value was adjusted to be 9.0.

EXAMPLE 18

After adding 40 g of (NH₄)₂ HPO₄ to the prescription in Example 8, by further adding succinic acid thereto, the pH value was adjusted to be 4.5.

EXAMPLE 19

After adding 10 g of methacrylic acid polymer to the prescription in Example 5, by further adding tartaric acid thereto, the pH value was adjusted to be 4.0.

COMPARATIVE EXAMPLE 1

    ______________________________________                                         sodium ferrocyanate      40     g                                              diammonium phosphate     20     g                                              water                    1000   ml                                             ______________________________________                                    

By adding citric acid to the above prescription, the pH value was adjusted to be 5.0.

COMPARATIVE EXAMPLE 2

    ______________________________________                                         phytic acid              50     g                                              gum arabic               1      g                                              water                    1000   ml                                             ______________________________________                                    

By adding NaOH to the above prescription, the pH value was adjusted to be 5.0.

COMPARATIVE EXAMPLE 3

    ______________________________________                                         tannic acid              20     g                                              water                    1000   ml                                             ______________________________________                                    

By adding NaOH to the above prescription, the pH value was adjusted to be 5.0.

COMPARATIVE EXAMPLE 4

    ______________________________________                                         monocalcium salt of phytic acid                                                                         40     g                                              phosphoric acid          65     g                                              NaOH                     50     g                                              water                    1000   ml                                             ______________________________________                                    

Next, after applying the respective treating liquids obtained as above to a commercial electrophotographic type-lithographic master prepared through the desensitizing process at a feed rate of 50 mm/sec. by means of RICOH ETCHING PROCESSOR, the manufacture of K. K. RICOH, offset printing was conducted. In this context, water was employed as wetting solution.

The result was as shown in the following table-1, respectively.

                                      TABLE-1                                      __________________________________________________________________________                          Condition of lithographic                                                                   85 lines/inch, 10-gradation                         Occurrence of stains of                                                                      plate after turning out                                                                     reproducibility when 1,000                          gear-stripe in printing                                                                      5,000 prints.                                                                               prints were turned out.                      __________________________________________________________________________     Example                                                                             1 No occurrence when 5,000                                                                     No stains at all.                                                                           8                                                   prints were turned out.                                                 "    2 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    3 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    4 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    5 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    6 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    7 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    8 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    9 No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    10                                                                               No occurrence when 10,000                                                                    "            9                                                   prints were turned out.                                                 "    11                                                                               No occurrence when 10,000                                                                    "            "                                                   prints were turned out.                                                 "    12                                                                               No occurrence when 15,000                                                                    "            "                                                   prints were turned out.                                                 "    13                                                                               No occurrence when 15,000                                                                    "            "                                                   prints were turned out.                                                 "    14                                                                               No occurrence when 10,000                                                                    "            "                                                   prints were turned out.                                                 "    15                                                                               No occurrence when 5,000                                                                     "            8                                                   prints were turned out.                                                 "    16                                                                               No occurrence when 5,000                                                                     "            "                                                   prints were turned out.                                                 "    17                                                                               No occurrence when 15,000                                                                    "            9                                                   prints were turned out.                                                 "    18                                                                               No occurrence when 15,000                                                                    "            "                                                   prints were turned out.                                                 "    19                                                                               No occurrence when 10,000                                                                    "            "                                                   prints were turned out.                                                 Comparative                                                                           Stains occurred upon                                                                         Stains on the coarsened                                                                     8                                            Example 1                                                                             turning out 1,000 prints.                                                                    surface                                                   Comparative                                                                           Stains occurred upon                                                                         Stains on the whole sur-                                                                    7                                            Example 2                                                                             turning out 50 prints.                                                                       face, as well as the                                                           coarsened surface                                         Comparative                                                                           Stains occurred upon                                                                         No stains, but remarkable                                                                   4                                            Example 3                                                                             turning out 300 prints.                                                                      collapse of image.                                        Comparative                                                                           Stains occurred upon                                                                         Stains on the coarsened                                                                     5                                            Example 4                                                                             turning out 300 prints.                                                                      surface                                                   __________________________________________________________________________

EXAMPLE 20

    ______________________________________                                         K[Co(NH.sub.3).sub.2 (NO.sub.2).sub.4 ]                                                                 30     g                                              water                    1000   ml                                             ______________________________________                                    

By adding tartaric acid to the above prescription, the pH value was adjusted to be 5.0.

EXAMPLE 21

    ______________________________________                                         Na[Co(NH.sub.3).sub.2 (NO.sub.2) (C.sub.2 O.sub.4)]                                                     40     g                                              water                    1000   ml                                             ______________________________________                                    

By adding phosphoric acid to the above prescription, the pH value was adjusted to be 4.5.

EXAMPLE 22

    ______________________________________                                         K.sub.3 [Co(C.sub.2 O.sub.4).sub.3 ]                                                                    20     g                                              (NH.sub.4).sub.2 HPO.sub.4                                                                              20     g                                              water                    1000   ml                                             ______________________________________                                    

By adding citric acid to the above prescription, the pH value was adjusted to be 4.5.

EXAMPLE 23

    ______________________________________                                         K[Co(edta)]              30     g                                              CMC                      2      g                                              water                    1000   ml                                             ______________________________________                                    

By adding adipic acid to the above prescription, the pH value was adjusted to be 4.5.

EXAMPLE 24

    ______________________________________                                         K.sub.3 [Ni(C.sub.2 O.sub.4).sub.3                                                                      25     g                                              NH.sub.4 H.sub.2 PO.sub.4                                                                               10     g                                              water                    1000   ml                                             ______________________________________                                    

By adding malic acid to the above prescription, the pH value was adjusted to be 5.0.

EXAMPLE 25

    ______________________________________                                         Na[Fe(edta)]             20     g                                              alginic acid             5      g                                              water                    1000   ml                                             ______________________________________                                    

By adding malonic acid to the above prescription, the pH value was adjusted to be 5.0.

Next, absorbent cotton was soaked with the respective treating liquids obtained as above, and by the use of the thus soaked cotton, a commercial zinc oxide-resin dispersion type electrophotographic printing plate prepared through electrophotographic process was desensitized and then served for printing. In this context, as the wetting solution, a solution obtained by diluting the respective treating liquids with water to increase fivefold was employed. The result was as shown in the follwing table 2.

                  TABLE-2                                                          ______________________________________                                                                85 lines/inch, 10-                                                             gradation reproduci-                                              Occurrence of stains                                                                        bility when 1,000                                                 of gear-stripe in                                                                           prints were turned                                                printing     out.                                                    ______________________________________                                                         No occurrence when                                             Example  20     10,000 prints were                                                                            9                                                               turned out.                                                                    No occurrence when                                             "        21     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             "        22     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             "        23     10,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             "        24     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             "        25     10,000 prints were                                                                            "                                                               turned out.                                                    ______________________________________                                    

EXAMPLE 26

    ______________________________________                                         [Co(NH.sub.2).sub.6 Cl.sub.3                                                                            50     g                                              diammonium phosphate     100    g                                              water                    1000   ml                                             ______________________________________                                    

By adding malonic acid to the above prescription, the pH value was adjusted to be 5.0.

EXAMPLE 27

By adding malic acid in place of malonic acid to the prescription in Example 26, the pH value was adjusted to be 5.0.

EXAMPLE 28

    ______________________________________                                         [Co(NH.sub.3).sub.6 ](CClO.sub.4).sub.3                                                                 50     g                                              disodium phosphate       10     g                                              water                    1000   ml                                             ______________________________________                                    

The pH value of the solution was 8.4.

EXAMPLE 29

By adding citric acid in place of malonic acid in Example 26, the pH value was adjusted to be 6.0.

EXAMPLE 30

    ______________________________________                                         [Co(NH.sub.3).sub.6 (NO.sub.3).sub.3                                                                    50     g                                              metaphosphoric acid      50     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 31

    ______________________________________                                         [Co(NH.sub.3).sub.6 ](CF.sub.3 COO).sub.3                                                               20     g                                              monoammonium phosphate   50     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 32

    ______________________________________                                         [Co(NH.sub.3).sub.6 ]SO.sub.4                                                                           50     g                                              molybdenum phosphate     100    g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 33

    ______________________________________                                         [Co(NH.sub.3).sub.6 ] (OH).sub.2 . 6H.sub.2 O                                                           30     g                                              hexaphosphoric acid      60     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 34

    ______________________________________                                         [Co(NH.sub.3).sub.6 ] (HPO.sub.4).sub.3 . 4H.sub.2 O                                                    50     g                                              phosphoric acid          60     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 35

By adding caustic soda to the solution in Example 34, the pH value was adjusted to be 4.5.

EXAMPLE 36

    ______________________________________                                         [Co(NH.sub.3).sub.6 ]I.sub.3                                                                            50     g                                              monomagnesium phosphate  50     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 37

50 g of phytic acid were added to the solution in Example 26.

EXAMPLE 38

1 g of sodium dehydroacetate was added to the solution in Example 26.

EXAMPLE 39

1 g of EDTA was added to the solution in Example 26.

EXAMPLE 40

50 g glycerine were added to the solution in Example 26.

EXAMPLE 41

    ______________________________________                                         [Ni(NH.sub.3).sub.6 ]Cl.sub.3                                                                           10     g                                              [Co(NH.sub.3).sub.5 (OH.sub.2)] (C.sub.2 O.sub.4).sub.1.5 . 2H.sub.2                                    10     g                                              (NH.sub.4).sub.2 HPO.sub.4                                                                              30     g                                              malonic acid             20     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 42

    ______________________________________                                         [Fe(NH.sub.3).sub.5 NO.sub.2 ]Cl.sub.2                                                                  10     g                                              Na[Co(NH.sub.3).sub.2 (NO.sub.2)(C.sub. 2 O.sub.4)]                                                     10     g                                              Na.sub.2 HPO.sub.4       30     g                                              citric acid              20     g                                              water                    1000   ml                                             ______________________________________                                    

EXAMPLE 43

    ______________________________________                                         [Co(NH.sub.3).sub.4 (NO.sub.2).sub.2 ]Cl                                                                10     g                                              K[Co(NH.sub.3).sub.2 (NO.sub.2).sub.4 ]                                                                 10     g                                              H.sub.3 PO.sub.4         30     g                                              NaOH                     10     g                                              water                    1000   ml                                             ______________________________________                                    

COMPARATIVE EXAMPLE 5

    ______________________________________                                         sodium ferrocyanate      50     g                                              diammonium phosphate     50     g                                              water                    1000   ml                                             ______________________________________                                    

By adding malonic acid to a solution prescribed as above, the pH value was adjusted to be 5.0.

When a variety of electrophotographic offset masters prepared through the desensitizing process by employing the respective treating liquids obtained as above and a direct image-printing type offset master (which was prepared by typewriting with a typewriter and thereafter drawing with a sign-pen charged with oily ink, a ball-point pen and an HB pencil) were subjected to etching and then served for offset printing while employing water as wetting solution, the result was as shown in the following table 3, respectively.

                  TABLE-3                                                          ______________________________________                                                                85 lines/inch, 10-**                                                           gradation reproduci-                                              Occurrence of stains                                                                        bility when 1,000                                                 of gear-stripe in                                                                           prints were turned                                                printing     out.                                                    ______________________________________                                                         No occurrence when                                             Example  26     15,000 prints were                                                                            9                                                               turned out.                                                                    No occurrence when                                             Example  27     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  28     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  29     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  30     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  31     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  32     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  33     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  34     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  35     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  36     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  37     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  38     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  39     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  40     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  41     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  42     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example  43     15,000 prints were                                                                            "                                                               turned out.                                                                    No occurrence when                                             Example   1*    3,000 prints were                                                                             "                                                               turned out.                                                                    No occurrence when                                             Example   2*    3,000 prints were                                                                             "                                                               turned out.                                                                    No occurrence when                                             Example   5*    3,000 prints were                                                                             "                                                               turned out.                                                                    No occurrence when                                             Example   6*    3,000 prints were                                                                             "                                                               turned out.                                                    Comparative     Stains occurred upon                                           Example 5       turning out 1,000                                                                             8                                                               prints                                                         Comparative     Stains occurred upon                                           Example 5*      turning out 100                                                                               "                                                               prints                                                         ______________________________________                                          *A direct imageprinting type offset master was used. In other examples, a      electrophotographic offset master was used.                                    **Reproducibility evaluated by a means for judging the reproducibility         which comprises forming a toner image of 85 lines per inch on a zinc           oxideresin dispersion type electrophotographic printing plate in               10gradation density, performing etching on the plate and then serving the      thus processed plate for printing, thereby judging the degree of fidelity      of the reproduced image. When the value is 8 or more, the reproducibility      is good, and when it is less than 8, the reproducibility is poor.         

What is claimed is:
 1. A method for desensitizing an offset printing plate which comprises applying an aqueous desensitizing composition consisting essentially of at least one complex to a non-image area of the offset printing plate having an oleophilic image formed thereon in an amount effective to desensitive said non-image area, said complex being selected from the group consisting of compounds having the formula I, [M(X₁)_(a) ](Y)_(b).cH₂ O, wherein M is selected from the group consisting of Ir, Co, Ti, Fe, Cu, Ni, Pt, Mn, Ru, Rh, Hf, V, Be and Pd, X₁ is NH₃, OH₂, H₂ N(CH₂)₂ NH₂, C₂ O₄, NO, NO₂, OCHO, NH₂, HONC(CH₃)C, (CH₃)NO, ##STR5## OCN₂ H₄ or OC(NH₂)₂, Y is an anion, "a" is a number in the range of from 2 to 6, "b" is a number in the range of from 1 to 3 and "c" is 0 or a number in the range of from 1 to 10, compounds having the formula II, [M(X₁)_(a), (X₂)_(a") ](Y)_(b).cH₂ O, wherein M, X₁, Y, "b" and "c" are respectively the same as in formula I, X₂ is OH, OH₂, NO₂, CO₃, NH₂ CH₂ COO, HONC(CH₃)C(CH₃)NO, Br, Cl, H₂ N(CH₂)₂ NH₂, N, ONO₂, ONO, NCS, H₂ O, ##STR6## F or I, and "a'" and "a"" are respectively a number in the range of from 1 to 5, compounds having the formula III, (M₁)_(p) [(M₂)(X₃)_(q) ].nH₂ O, wherein M₁ is Na, K, NH₄ or hydrogen, M₂ is selected from the group consisting of Ir, Co, Ti, Fe, Cu, Ni, Pt, Mn, Ru, Rh, Hf, V, Be and Pd, X₃ is C₂ O₄, NO₂, Cl, Br, I or ##STR7## "p" is a number in the range of from 1 to 3, "q" is a number in the range of from 1 to 6, and "n" is 0 or a number in the range of from 1 to 10, compounds having the formula IV, (M₁)_(p) [(M₂)(X₃)_(q) (X₄)_(r) ].nH₂ O, wherein M₁, M₂, X₃, "p", "q" and "n" are respectively the same as in the formula III, X₄ is NH₃ or NH₂ CH₂ CH₂ NH₂, and "r" is a number in the range of from 1 to 6, and compounds having the formula V, (M₁)_(p) [(M₂)(X₃)_(q) (X₄)_(r) (X₅)_(s) ].nH₂ O, wherein M₁, M₂, M₃, "p", "q" and "n" are respectively the same as in the formula III, X₄ and "r" are respectively the same as in the formula IV, X₅ is C₂ O₄, NO₂, Cl or Br, and "s" is a number in the range of from 1 to
 6. 2. A method according to claim 1 in which complex is hexamine cobalt complex.
 3. A method according to claim 1, wherein said complex is selected from the group consisting of [Co(NH₃)₆ ](HPO₄)₂.4H₂ O, [Co(NH₃)₆ ]PO₄.4H₂ O, [Co(NH₃)₆ ].(ClO₄)₃, [Co(NH₃)₆ ](OH)₃.6H₂ O, [Co(NH₃)₆ ]SO₄, [Co(NH₃)₆ ]Br, [Co(NH₃)₆ ]Cl₃, [Co(NH₃)₆ ](NO₃)₃, [Co(NH₃)₆ ]I₃, [Co(NH₃)₆ ]F₃, [Co(NH₃)₆ ](CF₃ COO)₃, [Co(NH₃)₆ ](CCl₃ COO)₃, [Co(NH₃)₆ ](ClO₃)₃, [Co(NH₃)₆ ]SO₄ Cl, [Co(NH₃)₆ ](TiCl₆), [CO(NH₃)₆ ](BiCl₆), [Fe(NH₃)₆ ]I₂, [Ti(NH₃)₄ Cl₂ ]Cl, [ Mn(NH₃)₆ ]Cl₂, [Co(NH₃)₅.H₂ O]Br₃, [Ru(NH₃)₆ ]₂ (SO₄)₁.5.2.5H₂ O, {Cu[H₂ N(CH₂)₂ NH₂ ]₃ }Cl₃, [Pt(NH₃)₆ ](OH)₄, [Ni(NH₃)₆ ](ClO₃)₂, [Co(NH₃)₄ Cl₂ ]Cl, [Fe(NH₃)₅ NO₂ ]Cl₂, [Co(NH₃)₅ (OH₂)]C₂ O₄)₁.5.2H₂ O, [Ni(NH₃)₆ ](ClO₃)₂, [Co(NH₃)₄ (NO₂)₂ ]Cl, [Mn(NH₃)₆ ]Cl₃, [Fe(NH₃)₆ ]I₂, K[Co(NH₃)₂ (NO₂)₄ ], Na[Co(NH₃)₂ (NO₂)₄ ], NH₄ [Co(NH₃)₂ (NO₂)₄ ], K[Co(NH₃)₂ (NO₂)₂ (C₂ O₄)].H.sub. 2 O, Na[Co(NH₃)₂ (NO₂)₂ (C₂ O₄)].H₂ O, NH₄ [Co(NH₃)₂ (NO₂)₂ (C₂ O₄)].H₂ O, Na₃ [Co(C₂ O₄)₃ ], (NH₄)₃ [Co(C₂ O₄)₃ ], K₃ [Co(C₂ O₄)₃ ], Na₃ [Co(NO₂)₆ ], (NH₄)₃ [Co(NO₂)₆ ], K[Co(edta)], Na[Co(edta)], (NH₄)[Co(edta)], K₃ [CoCl₆ ], Na₃ [CoCl₆ ], (NH₄)₃ [CoCl₆ ], K₃ [CoBr₆ ], Na₃ [CoBr₆ ], (NH₄)₃ [CoBr₆ ], K[Co(NH₂ CH₂ CH₂ NH₂)(NO₂)₄ ], Na[Co(NH₂ CH₂ CH₂ NH₂)(NO₂)₄ ], NH₄ [CO(NH₂ CH₂ CH₂ NH₂)(NO₂)₄ ], K[Co(NH₂ CH₂ CH₂ NH₂)₂ (NO₂)₂ ], Na[Co(NH₂ CH₂ CH₂ NH₂)₂ (NO₂)₂ ], NH₄ [Co(NH₂ CH₂ CH₂ NH₂)₂ (NO₂)₂ ], K₃ [Ni(C₂ O₄)₃ ], Na₃ [Ni(C₂ O₄)₃ ], (NH₄)₃ [Ni(NO₂)₆ ], K₂ [Ni(edta)], Na₂ [Ni(edta)], (NH₄)₂ [Ni(edta)], K₂ [Fe(edta)], Na[Fe(edta)], (NH₄)[Fe(edta)], K₃ [Fe(C₂ O₄)₃ ], Na₃ [Fe(C₂ O₄)₃ ], (NH₄)₃ [Fe(C₂ O₄)₃ ], Pt[Pt(NH₃)₄ Cl₂ ], H₂ [PtCl₆ ], K₂ [PtCl₆ ], K₂ [PtI₆ ], H₂ [Pt(NO₂)₄ ], K₂ [Pt(NO₂)₄ ], Na₂ [Pt(NO.sub. 2)₄ ], K₂ [Pt(C₂ O₄)₂ ], Na₂ [Pt(C₂ O₄)₂ ], NH₄ [Co(NH₃)₂ (C₂ O₄)(NH₂ CH₂ CH₂ NH₂)], Na[Co(NH₃)₂ (C₂ O₄)(NO₂)₂ ], NH₃ [Co(NH₃)₂ (C₂ O₄)(NO₂)₂ ], (NH₄)₂ [Pt(C₂ O₄)₂ ], K₂ [Pd(NO₂)₄ ], Na₂ [Pd(NO₂)₄ ], (NH₄)₂ [Pd(NO₂)₄ ], K₂ [Pd(C₂ O₄)₂ ], Na₂ [Pd(C₂ O₄)₂ ] and (NH₄)₂ [Pd(C₂ O₄)₂ ], wherein "edta" is ##STR8##
 4. A method according to claim 2, wherein said hexamine cobalt complex is selected from the group consisting of [Co(NH₃)₆ ]Cl₃, [Co(NH₃)₆ ](HPO₄)₃.4H₂ O, [Co(NH₃)₆ ]PO₄.4H₂ O, [Co(NH₃)₆ ].(ClO₄)₃, [Co(NH₃)₆ ](OH)₃.6H₂ O, [Co(NH₃)₆ ]SO₄, [Co(NH₃)₆ ]Br, [Co(NH₃)₆ ](NO₃)₃, [Co(NH₃)₆ ]I₃, [Co(NH₃)₆ ]F₃, [Co(NH₃)₆ ](CF₃ COO)₃, [Co(NH₃)₆ ](CCl₃ COO)₃, [Co(NH₃)₆ ](ClO₃)₃, [Co(NH₃)₆ ]SO₄ Cl, [Co(NH₃)₆ ](TiCl₆) and [Co(NH₃)₆ ](BiCl₆).
 5. A method according to claim 1, wherein the concentration of said complex is in the range of from 0.1 to 30 weight percent.
 6. A method according to claim 1, wherein said desensitizing composition contains at least one assistant selected from the group consisting of phosphate and inorganic acid.
 7. A method according to claim 6, wherein said inorganic acid is phosphoric acid.
 8. A method according to claim 6, wherein said assistant is phosphate.
 9. A method according to claim 2, wherein said desensitizing composition contains at least one assistant selected from the group consisting of phosphate and inorganic acid.
 10. A method according to claim 9, wherein said inorganic acid is phosphoric acid.
 11. A method according to claim 9, wherein said assistant is phosphate.
 12. A method according to claim 2, wherein the concentration of said complex is in the range of from 0.1 to 30 weight percent.
 13. A method according to claim 4, wherein the concentration of said hexamine cobalt complex is in the range of 0.1 to 30 weight percent, and said assistant is selected from the group consisting of phosphoric acid, metaphosphoric acid, hexaphosphoric acid, trimetaphosphoric acid, dodecaoxo-6-phosphoric acid, hypophosphoric acid, monoammonium phosphate, diammonium phosphate, triammonium phosphate, monosodium phosphate, disodium phosphate, trisodium phosphate, monopotassium phosphate, dipotassium phosphate, tripotassium phosphate, phosphomolybdic acid, sodium pyrophosphate, ammonium phosphomolybdate, monocalcium phosphate, monomagnesium phosphate, sodium ammonium phosphate, imidometaphosphoric acid, calcium pyrophosphate, the amount of said assistant being in the range of from 0.1 to 20 parts by weight, per one part by weight of said hexamine cobalt complex.
 14. A method according to claim 1, wherein is I, Br, Cl, Cl₃, Cl₄, C₂ O₄, SO₄, NO₃, NO₂, CH₃, COO, HCOO, BF₄, MnO₄, OH, F, HSO₄, HPO₄, PO₄, HPO₃ and SO₄ X, wherein X is Cl, Br, I, ClO₄ or NO₃.
 15. A method according to claim 9 wherein the concentration of said assistant is in the range of from 0.1 to 20 parts by weight, per one part by weight of said hexamine cobalt complex. 